1. Field of the Invention
The present invention relates generally to cathode ray tubes and more particularly is directed to a cathode ray tube in which two electron beams simultaneously scan a phosphor screen which are spaced a distance of approximately half the spacing between adjacent scanning lines in the vertical direction.
2. Description of the Prior Art
In general, in the display on a picture screen by the interlaced system, when there are 525 scanning lines, one field is formed of 262.5 scanning lines, which then is transmitted at a frequency of 60 Hz so that field flicker is suppressed. Moreover, in order to obtain the vertical resolution, the field next to a first field is scanned with a displacement corresponding to half the distance between adjacent scanning lines.
In this case, although microscopically the number of images is 60 sheets/sec, microscopically one scanning line is scanned every 1/30 second and its display period is 1/30 second. Therefore, the scanning by one scanning line can result in flicker. In other words, line flicker exists.
In order to reduce the line flicker, it is sufficient to shorten the display period of one scanning line to less than 1/30 second.
In order to solve this problem, a television receiver which employs a cathode ray tube of the 2-beam system may be considered. A first electron beam Bm1 and a second electron beam Bm2 scan simultaneously the scanning lines on the picture screen with a distance of half the spacing between adjacent scanning lines in the vertical direction. FIGS. 1B and 1C illustrate scanning states of first and second electron beams Bm1 and Bm2 on a picture screen 100 for odd and even fields, respectively. Fig. lA shows the scanning state for an electron beam Bm in the case of 1-beam system.
When there are 525 scanning lines, in the case of a 1-beam system, only 262.5 scanning lines are scanned within one field, while in the case of a 2-beam system, the remaining 262.5 scanning lines which will be scanned during the next field are scanned by, for example, the second electron beam Bm2 and then scanned so that 525 scanning lines can all be scanned within one field. Thus, the display period for each scanning line becomes 1/60 second, thereby removing line flicker.
For the above mentioned cathode ray tube of the 2-beam system, there has been proposed a cathode ray tube with the first and second cathodes for the first and second electron beams Bm1 and Bm2 disposed parallel to each other in the vertical direction. This previously proposed cathode ray tube, however, has the following defects.
For a deflection yoke for the cathode ray tube of the 2-beam system, in view of the convergence at respective portions of the picture screen, a deflection yoke of the CFD (convergence free deflection yoke) type may be desired. In this type of deflection yoke, the horizontal deflection coil is formed of a saddle winding, while the vertical deflection coil is formed of a toroidal winding, and thus the vertical deflection magnetic field will be extended primarily to the side of the tube neck. As a result, the deflection in the vertical direction is large. When the first and second cathodes are disposed parallel to each other in the vertical direction, in a cathode ray tube of the Trinitron (registered trademark) type, deflection plates 1 are disposed one on the other in the vertical direction y as shown in FIG. 2. Accordingly, in the case of the Trinitron type tube in the deflection yoke of the CFD type, there is a problem in that the electron beams Bm1 and Bm2 may strike the deflection plates 1. Therefore, the deflection plates 1 must be disposed at a position which is free from the influence of the vertical deflection magnetic field, thus increasing the length of the envelope of the cathode ray tube.
Moreover, in the deflection yoke of CFD type, usually, the horizontal deflection magnetic field is formed as a pin-cushion type as shown in FIG. 3A. Therefore, a horizontal deflection coil C.sub.H has a winding distribution such as shown in FIG. 4A, and the winding density thereof becomes lower at a position nearer to the axis y (in the vertical direction). To obtain such winding distribution, the horizontal deflection coil C.sub.H is manufactured by using a metal mold 2 as shown in FIG. 5A. In this case, the amount of wire material 3 which is wound deep in the metal mold 2 is small and the winding thereof is relatively easy and hence the accuracy during manufacturing is easy to obtain.
On the other hand, when the first and second cathodes are disposed parallel to each other in the vertical direction, the horizontal deflection magnetic field must be formed as a barrel type as shown in FIG. 3B. Therefore, for this type, the horizontal deflection coil C.sub.H must have a winding distribution such as shown in FIG. 4B, and the winding density thereof becomes higher at positions nearer to the axis y. To obtain the winding distribution shown in FIG. 4B, the horizontal deflection coi1 C.sub.H is manufactured by using a metal mold 2' as shown in FIG. 5B. Accordingly, in this case, the amount of the wire material 3 which is wound deep in the metal mold 2' is large, and it is difficult to wind and accuracy during manufacturing is difficult to obtain. In FIGS. 3 and 4, x represents the horizontal direction.
Furthermore, in the deflection yoke of CFD type, when the first and second cathodes are disposed parallel to each other in the vertical direction, the horizontal deflection magnetic field must be formed as the barrel type magnetic field as described above. However, this causes the beam spot shape F.sub.BM on a phosphor screen 4' to become long in the longitudinal direction at its periphery as shown in FIG. 6. Thus, the scanning lines overlapped each other, causing deterioration in the vertical resolution.